Superheterodyne receiver



April 21, 1936. G. L. BEERS SUPERHETERODYNE RECEIVER Filed Deo. 29, 1955 www NW nu Patented Apr. 2l, 1 936 UNITED STATES SUPERHETERODYNE RECEIVER George L. Beers, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 29, 1933, Serial No. 704,437

7 Claims. (Cl. Z50-20) My invention relates to radio receivers and particularly to receivers of the superheterodyne type.

As explained somewhat in detail in my Patent 1,915,483, superheterodyne receivers are subject to various types of interference such as image frequency response, reception of signals heterodyned by harmonics of the local oscillator frequency, and the simultaneous reception of signals from two stations, the carriers of which differ in frequency by the intermediate frequency. Many expedients have been resorted to for eliminating or reducing such interference, the above-mentioned patent being an example. The most commonly employed expedient is the utilization of a selecting circuit or network, such as a tuned radio frequency amplifier, between the antenna and the first detector. Present practice is to make this selecting circuit as sharply tuned as possible consistent with the Acommunication band it is desired to receive, both for the purpose of reducing image frequency response and similar interference to a-minimum, and for the .purpose of increasing the over-all selectivity of the receiver.

In a uni-controlled receiver, this requires the simultaneous tuning of a plurality of sharply tuned circuits, and it has been found diiiicult to keep the various tuning devices properly aligned. It has been found especially difficult to make the oscillator trackwith the sharply tuned circuits.

By tracking is meant causing the oscillator frequency to continuously differ from the tuning of the tuned circuits by' a fixed value equal to the intermediate frequency as the` receiver is tuned over its frequency range.

An object of my invention is to provide an improved superheterodyne receiver that shall be free from interference of the above mentioned types.

A further object of my invention is to provide an improved superheterodyne receiver employing variable inductance tuning.

A still further object of my invention is to pro- .x vide an improved method and meansfor preventing interference, of the above-mentionedv types, in a superheterodyne receiver Without introducing difficulties in aligning the tuning devices of the receiver.

In practicing my invention, I provide a superheterodyne receiver with a preselecting circuit in the form of a variable band-pass filter having a pass range which is narrow enough to prevent the various forms of superheterodyne interference, but which is so wide that there can be no diiculty in aligning the tuning devices. The pass range is also so wide that it does not contribute to the adjacent channel selectivity of the receiver. In a preferred embodiment of my invention, the band-pass lter comprises a plurality of variable iron-core inductances, these inductances being variable simultaneously with the tuning of the oscillator. I

By the "pass range of the above-mentioned variable band-pass filter is meant that frequency range or band which lies between the upper and lower cut-off points of the filter at any given filter setting. This definition also applies to any tuned circuit or system designed to pass a band of frequencies for a given adjustment or setting thereof. The pass" range of a circuit may also be defined as the frequency band Within which the circuit will pass signals with minimum attenuation.

Other objects, features, and advantages of my invention will appear from the following description, taken in connection with the accompanying drawing, in which Fig. 1 is a schematic diagram of a radio receiver embodying my invention;

Fig. 2 is a view showing the construction of a portion of the apparatus shown in Fig. 1; and

Fig. 3 is a set of curves showing certain characteristics of my improved receiver.

Referring to Fig. 1, the radio receiver comprises a first detector I having an input circuit which is coupled to an antenna 3 through a variable band-pass filter indicated generally at 5. The input circuit of the first detector includes a resistor 'l which is the terminating impedance of the band-pass filter. A coupling coil Sand a self-biasing resistor II are connected in series with the terminating impedance, the self-biasing resistor I I being shunted in the usual manner by by-pass condenser I3. A pentode tube such as the type known as the RCA-57 has been found satisfactory for use as the first detector.

Inorder to convert incoming radio frequency signals to a predetermined intermediate frequency, an oscillator I5 is provided which is coupled tothe first detector I through the coupling coil 9. The frequency of the oscillator output is varied by means of a variable inductance coil I'I in the tuned oscillator circuit as will be more fully described hereinafter. The oscillator feed-back coil I6 and the coupling coil 9 are preferably wound over the vcathode or low potential end of the coil I'I.

The intermediate frequency signal output of detector I is fed to an intermediate frequency amplifier I9 through an intermediate frequency transformer 2|. The transformer 2I is tuned by means of condensers 23 and 25 shunting the primary and the secondary windings, respectively, to function as a band-pass filter having a pass" range of sufficient width to pass the communication band, which band may be the carrier and both side bands or the carrier and only one side band.

'I'he output of the intermediate frequency amplilier I9 may be fed to another amplifier or to a second detector through an intermediate frequency transformer 21 tuned to function as a band-pass lter in the same manner as the transformer 2|.

The variable band-pass filter 5 forms a pre- Y selecting circuit which is-provided solely for the purpose of preventing the various types of superheterodyne interference previously noted. While the band-pass filter is referred to as a preselecting circuit, it does not preselect in the sense that it selects the particular station to be received. It merely selects a frequency band within which there is no signal that can be heterodyned to produce an interfering signal. All adjacent channel selectivity is provided by the superheterodyne oscillator I5 and the tuned intermediate frequency transformers 2| and 21. That is, the actual station selecting is done by varying the frequency of the oscillator output. This will be more clearly understood'by referring to specific values which are employed in one receiver embodying the invention. In this particular receiver, the intermediate frequency is 1'75 kilocycles and the pass range of the band-pass filter 5 is 75 kilocycles.

Since this particular receiver is designed to receive radio signals having a communication channel of approximately 10 kilocycles in width, the pass range of the intermediate frequency transformers is made 10 kilocycles wide. It will be understood, of course, that where the communication channel is wider, as in the case of high fidelity transmission, the pass range of the intermediate frequency transformers will be correspondingly greater to permit high fidelity reception. For example, they may have a pass range of 20 kilocycles.

It will be evident that with a pass range of the order of '75 kilocycles, the band-pass filter can not contribute to the adjacent channel selectivity Where the frequency spacing of adjacent carrier Waves is 10 kilocycles as it is in the broadcast band. On the other handsince the pass range of the filter is less than ,the intermediate frequency of the receiver, it lwill prevent image respense and the various other types of superheterodyne interference hereinbefore mentioned. For example, two incoming signals can beat together in the first detector to produce an interfering signal at the intermediate frequency only if the filter passes signals differing in frequency by an amount equal to the intermediate frequency. If the filter is designed to have such attenuation characteristics as to substantially prevent the passage of two signals differing in frequency by an amount equal to the intermediate frequency, it' will also prevent any substantial image frcquency response providing the cut-off of the filter is sharp enough, it being necessary that the undesired signal be attenuated more for keeping image frequency interference at, a low value than for keeping the other forms of interference at a low value.

Referring now to the design of the band-pass filter 5, it comprises a plurality of filter sections,

each section including a tunable series inductance coil having inductance Li and identified by the letter L1, and a tunable circuit connected in shunt thereto. The vtunable shunt circuit' includes a condenser Cz, having capacity Cz (530 micro- 5 microfarads in the particular receiver being described), connected in parallel with an inductance coil having a value L: and identified by the letter La. The band which the filter passes is determined by the position oi a plurality of movable 'cores 29 and 3i, respectively, which change the inductance values of the coils as they are moved with respect thereto. Each movable core 3| comprises, in addition to an iron core 33, a copper tube 35 fastened to the end of the iron core 33 for providing a wider change in the inductance. 'I'his feature of the invention will be more fully explained hereinafter, in connection with Fig. 2.y In the specific embodiment being described, the coils L1 are variable between 447 and 1020 micro-henries and the coils L: are variable between 26.2 and 158 micro-henries.

In accordance with the usual filter practice, the reactance units of the terminating shunt circuits are given twice the impedance values of the reactance units of the other shunt circuits in order to properly terminate the filter. The output end of filter 5 is terminated by the resistor 1, which, in the above-mentioned embodiment of the invention, has a resistance value of 4000 ohms, this also being the terminal impedance of the filter.

A variable inductance coil L: is provided for tuning the antenna and in the present embodiment is variable between 100 micro-henries and '755 micro-henries. The input end of the filter is 3 connected across variable inductance coil La, this connection preferably being made through a resistor 31 which, in the above-mentioned embodiment, has a value of 2000 ohms. 1

The antenna coil La, the filter coils L1 and La, and the oscillator coil I1 may be adjusted simultaneously by means of a common control device indicated at 39.

In order to make the receiver tune properly when connected to different antennas, a variable 4 condenser 4I is connected in series with the antenna. If the receiver is adjusted to tune properly when employing a certain antenna with the condenser set at a certain value, the condenser will be adjusted to a larger value oi capacity for proper tuning, if the receiver is connected to an antenna having a smaller capacity than the first mentioned antenna.

The filter structure will be better understood by referring to Fig. 2. In this figure, only a por- 5 tion of the variable inductance coils have been shown in order to simplify the drawing. Preferably, all the cores of these inductance coils are supported from a suitable supporting structure, such as a plate 43. The inductance coils L1, In, L3 and I1 are mounted underneath the movable cores so that the cores may be moved into or out of the coils by moving the plate 43 up or down. The position of plate 43 may be controlled in any lsuitable manner, as by means of a cam and slow 6 motion mechanism (not shown).

The cores 3| comprise the section 33 of magnetic material in the form of a cylindrical rod and the tube 35 of conducting material such as copper fastened to the end of the magnetic core section. This type of core is utilized in order to obtain greater change in the inductance values of the coils than possible with the iron core alone, the inductance value being minimum when the copper tube is completely inside the coil and maximum when the iron core section is completely inside the coil. The change in inductance required for the series-filter coils L1 is not so great as that required for the shunt coils so that the use of iron cores alone has been found to be satisfactory.

The magnetic material for the above-described cores preferably consists of finely comminuted iron, the small iron particles being held together by a binder having insulating properties for reducing eddyl current losses.

The value of the oscillator inductance coil I1 may be varied by means of a core 45 supported from the plate 43 and .movable simultaneously with the cores of the filter coils and antenna tuning coil. The core comprises a cylindrical rod 41 of magnetic material and a cylindrical member 49 of conducting material such as brass or copper fastened to the end thereof.

It is desirable to have the frequency of the oscillator change in such a wayas the receiver is tuned over the frequency range thereof that the oscillator frequency will always be higher than the midpoint frequency of the filter pass range by an amount equal to the intermediate frequency. As previously mentioned, this is referred to as causing the oscillator to track with the selecting circuit. In order to make the oscillator frequency change in this manner, the core of the oscillator inductance coil is specially shaped, whereby the inductance of the oscillator tuned circuit (consisting of coil I1 and a condenser 48) changes in the required manner as the receiver is tuned over the frequency range for which it is designed;

In the particular apparatus illustrated, the magnetic portion 41 of the core is cylindrical, the same as the other magnetic cores, while the brass section 49 is turned down to a reduced diameter near the point Where it is attached to the portion 4l so that this brass section is substantially conical in shape.

The above described core shape for the oscillator inductance coil is made such that the oscillator will track with an ideal filter. As a matter of fact, it is very difficult to make an oscillator track perfectly with a filter as filters are constructed in a factory under quantity production conditions. In production, there will be certain variations in the inductance of the filter coils, and other variations in the filter circuit which will cause the pass range of the lter to vary, as the receiver is tuned over its frequency range, in a Way which can not be readily controlled.

Therefore, it is one feature of my invention to make the fpass range of the filter so wide that such variation in the filter characteristics will not cause any detrimental effect on the operation of the receiver. This will be better understood by referring to Fig. 3.

Fig. 3 illustrates one case where a receiver embodying my invention is tuned to a signal at the low end o-f the broadcast band and another case where the receiver is tuned to a signal near the upper end of the broadcast band.

In the first case illustrated, the receiver is tuned to a signal having a carrier frequency of 625 kilocycles, that is, this signal is being heterodyned to the intermediate frequency signal of 175 kilocycles by the oscillator frequency of 800 kilocycles. It will be noted that the band-pass filter 5 is so adjusted with respect to the oscillator I5 that the signal, which is being received, falls at the midpoint of the pass range of filter l. the filter characteristic being indicatedA by the curve A.

Referring, now, to the second case, the receiverA However, in spite of the factthat ythe tuning of the oscillator has changed 10 or 20 kilocycles with respect to the mid-point of the filter pass range, the signal is received perfectly Without any side-band attenuation. It will be understood that the shift of the midpoint of the filter pass range might be in the opposite direction to the one illustrated in Fig. 3; or the midpoint of the pass range may shift with respect to the received signal in an irregular fashion, at one time being below the signal frequency and another time being above the signal frequency.

In some cases it maybe desirable to extend the effective tuning range of filter 5 by so adjusting it with respect to oscillator I5 that a signal being received at the low end of the frequency band falls on the low frequency sid-e of the midpoint of the pass range of filter 5 (instead of at the midpoint), While a signal at the high end of the frequency band falls on the high frequency side of said midpoint.

Attention is called to the fact that I have chosen a filter design such that the cut-olf characteristic of the filter at the end .of the pass range which is adjacent to the Ioscillator fre# quency is sharper than the cut-off characteristic at the other end of the pass range. In the receiver illustrated, since the oscillator frequency is higher than the signal frequency, the cut-off of the filter at the high frequency end of thel One advantage of my receiver design over that.

of conventional superheterodyne design will be apparent from a comparison of the curves A and C in Fig. 3, where curve C shows the selectivity of the selecting or radio frequency amplifier circuit in a conventional superheterodyne receiver. It will be Seen that the conventional selecting circuit is tuned rather sharply, having a pass range of only about 10 kilocycles, A pass range of 10 kilocycles is, of course, the minimum "pass range permissible for a circuit Which does not cut off any of the signal side-bands where the communication channel is l0 kilocycles in width.

A selecting circuit, having the characteristic shown by curve C contributes to the adjacent channel selectivity of the receiver but it also greatly increases the difficulty in tracking the oscillator with the selecting circuit. For example, if the oscillator frequency shifts even 2 or 3 kilocycles with respect to the midpoint of the resonant curve C in tuning from one end of the broadcast band to the other end, the receiver does not operate properly as the selecting circuit attenuates aportion of the communication band. Furthermore, a plurality of selecting circuits, tuned to give the overall characteristic shown by curve C, are difficult to align properly, as previously mentioned.

There is a further advantage in my receiver design in that a variable band-pass filter having variable inductanee coils is much easier to construct when its pass range is wide as specified in the foregoing description. One reason for this is that the individual parts of the filter may depart somewhat from their preferred relationship as the filter is being tuned, and the resulting irregularities in the filter characteristics will not be detrimental. Another reason is that, by making the pass range wide, the inductanee coils (especially the series coils) need not be given unreasonably large values.

From the foregoing description, it will be apparent that various modifications may be made in my invention 4wlthout departing from the spirit and scope thereof, and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and are set forth in the appended claims.

I claim:

l. In a radio receiver of the superheterodyne type, a signal collecting means, a first detector, a circuit tunable over a predetermined frequency range coupling said signal collecting means and said detector, an oscillator coupled to said first detector for heterodynlng an incoming signal to a signal having an intermediate frequency, said oscillator including a tunable circuit comprising an inductance coil having a core which is movable with respect thereto, and a uni-control means for tuning said first circuit and for causing relative movement between said coil and said cere simultaneously whereby thetunlng of said circuit and the frequency of the oscillator output may be varied simultaneously, said core being so shaped that over said frequency range the frequency of the oscillator output always differs from the tuning of said first circuit by an amount substantially equal to said intermediate frequency.

2. In a radio receiver, a radio frequency system coupled thereto and tunable throughout a predetermined frequency range, means including an oscillator for converting a radio frequency signal to an intermediate frequency signal, said converting means including a tunable circuit comprising a variable inductance coil having a core consisting of a magnetic section and a non-magnetic section of conducting material, an intermediate frequency system tuned to pass said intermediate frequency signal, said radio frequency system comprising a multi-section bandparss filter, each filter section including a variable inductanee coil having a core consisting of a magnetic section and a non-magnetic section of conducting material, and a uni-control means for varyingv said filter and said first variable inductance coil, said first core being so shaped that over said frequency range the frequency of the oscillator output always differs from the tuning of said radio frequency system by an amount sub stantially equal to said intermediate frequency.

3. In a superl'eterodyne receiver, a radio frequency system comprisinga band-pass filter tunable over a predetermined frequency range. means including a tunable oscillator for converting a radio. frequency signal to an intermediate frequency signal, and an intermediate frequency system tuned to pass said intermediate frequency signal, said filter including tuning means comprising series and shunt inductanee coils provided with variable cores, and common means for moving said cores simultaneously in predetermined relation to each other to pass a band of frequencies which is wide compared with the band of frequencies passed by said intermediate frequency system, and means in said filter providing a cut-off characteristic which is sharper at that end of the pass rangeof the lfilter which is the closer to the frequency of said oscillator.

4. 'I'he invention according to claim 1 characterized in that said core consists of a magnetic core section and a non-magnetic core section of a material which is an electrical conductor, said last section being tapering in cross section.J

5. The invention according to claim 1 characterized in that said core consists of a magnetic core section and a non-magnetic core section of material which is an electrical conductor, said last section being substantially conical in shape with its smaller end adjacent to said magnetic section.

6. The invention according to claim 2 characterized in that the non-magnetic section of said first core is of non-uniform cross-section.

7. In a superheterodyne receiver, a radio frequency circuit tunable throughout a predetermined frequency range and an intermediate frequency circuit, said radio frequency circuit including a multi-section band pass filter designed to pass a band of frequencies which is wide with respect to the band of frequencies passed by said intermediate frequency circuit, each section of said filter comprising a series inductanee coil, a shunt inductanee coil, and a condenser connected across said shunt inductanee coil, each of said series inductanee coils having a magnetic core and each of said shunt inductanee coils having a core comprising a magnetic section and non-magnetic section of conducting material, each inductance coil and magnetic core being relatively movable, means including an oscillator for converting a radio frequency signal to an intermediate frequency signal, said oscillator including a tunable circuit comprising a variable inductance coil having a core consisting of a magnetic section and a non-magnetic section of conducting material, uni-control means for producing relative movement of said inductanee coils and said cores simultaneously, said last-named core being so shaped that over said frequency range the frequency of the oscillator output always differs from the tuning of said radio frequency circuit by an amount substantially equal to said intermediate frequency.

GEORGE L. BEERS. 

